The present invention relates to a polyester ether film, and particularly it relates to a film comprising an aromatic polyester ether copolymer.
Since polyethylene terephthalate films and polytetramethylene terephthalate films are excellent in heat resistance, electrical insulating properties, gas barrier properties and scent retainability, they have been used for various applications by virtue of their advantageous properties, but they also have demerits such as poor flexibility, vulnerability to brake and liability to form pinholes. A solution to this problem is to blend flexible materials such as polyester elastomers in the film, but such flexible materials generally have poor compatibility with the film composition and cause whitening or opacification of the film even addition of a small quantity, so that it is unable to attain the desired flexibilization by addition of these materials. Also, the conventional terephthalic polyester films can hardly be used singly as a packaging material because of their high melting point and poor heat sealability, so that in order to make a satisfactory packaging material, it needs to form a composite film with a heat sealable resin such as polyethylenes through the medium of a costly adhesive.
A method for improving heat sealability of terephthalic polyester films is to use polyester films in which constituting polyester is produced by copolymerizing combinations of dicarboxylic acids other than terephthalic acid and glycols other than tetramethylene glycol and ethylene glycol, but these polyester films, which are made of amorphous polyesters, are subject to deterioration of gas barrier properties and scent retainability inherent to the normal terephthalic polyester films, and also lack flexibility.
As a result of present inventors' earnest studies to solve the above problem, it has been found that the above problem can be solved by specifying the percentage of the polytetramethylene oxide glycol residues in a polyester ether comprising as main components terephthalic acid, tetramethylene glycol and polytetramethylene oxide glycol. The present invention has been attained on the basis of the above finding.